The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates, are usually incorporated into the fluid composition to control viscosity.
Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to the driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications. Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in hotter and deeper wells under a variety of temperature and pH conditions. In these operations the rate of cross-linking is critical to the successful generation of viscosity. Frequently the rates of cross-linking with known cross-linking compositions are unacceptable, and new, highly specific compositions are required.
Commercially available zirconate cross-linkers, such as tetra-triethanolamine zirconate cross-link too fast under high pH (pH 10) conditions, causing a significant loss in viscosity due to shear degradation. Other zirconium complexes of triethanolamine, such as those disclosed in U.S. Pat. Nos. 4,578,488, 4,683,068, and 4,686,052 can be used as cross-linking agents. However, these complexes also do not cross-link at a desirable rate, especially at high pH conditions, causing a similar loss in viscosity due to shear degradation.
U.S. patent application Ser. No. 11/643,120, filed Dec. 21, 2006, discloses addition of 1 to 20 moles of water per mole of zirconium to a triethanolamine zirconate complex under certain conditions forms a stable complex with a 3-8 minute cross-linking rate, while maintaining satisfactory viscosity development. These cross-linkers have been found desirable for high temperature operations (149-177° C., 300-350° F.) because of the high initial viscosity they develop, but may be too slow for low temperature operations (121-149° C., 250-300° F.) and/or may not generate sufficient initial viscosity.
U.S. Pat. No. 4,579,670 discloses a general method of controlling reaction rates in a water based polymer fracturing fluid using a mixture of cross-linker in combination with a cross-linking rate retarder at a ratio such that the cross-linking reaction rate is controlled. The cross-linker employs a transition metal such as titanium, zirconium, chromium or hafnium. Triethanolamine and ethylene glycol are cited as rate retarders.
A glycol may be used as part of a cross-linked fluid or gel, but not as part of the cross-linker itself. U.S. Pat. Appl. 2006/0027364 discloses a method of treating subterranean formations using an aqueous gelled fluid comprising an aqueous fluid, a cross-linked guar gelling agent and an amount of a glycol such as ethylene glycol effective to increase stability of the fluid as measured by its viscosity, typically an amount of about 1 to about 10 volume % glycol based on the aqueous fluid. The glycol is added to the already cross-linked composition. US Pat. Appl. 2006/0264334 discloses the use of polyols such as ethylene glycol as a solvent to dissolve polymers used in fracturing fluids, not as part of the cross-linker itself.
The need exists for a cross-linker which develops high initial viscosity and which possesses a desirable 3-8 minute rate of cross-linking rate over a broad temperature range (121-177° C., 250-350° F.) for use in high pH (about pH 10 and above) fracturing fluids. The present invention meets these needs.